Overview of JSX and DOM in Deno

One of the common use cases for Deno is to handle workloads as part of web applications. Because Deno includes many of the browser APIs built-in, there is a lot of power in being able to create isomorphic code that can run both in Deno and in the browser. A powerful workload that can be handled by Deno is performing server side rendering (SSR), where application state is used server side to dynamically render HTML and CSS to be provided to a client.

Server side rendering, when used effectively, can dramatically increase the performance of a web application by offloading heavy calculations of dynamic content to a server process allowing an application developer to minimize the JavaScript and application state that needs to be shipped to the browser.

A web application is generally made up of three key technologies:

JavaScript
HTML
CSS

As well as increasingly, Web Assembly might play a part in a web application.

These technologies combine to allow a developer to build an application in a browser using the web platform. While Deno supports a lot of web platform APIs, it generally only supports web APIs that are usable in a server-side context, but in a client/browser context, the main "display" API is the Document Object Model (or DOM). There are APIs that are accessible to application logic via JavaScript that manipulate the DOM to provide a desired outcome, as well as HTML and CSS are used to structure and style the display of a web application.

In order to facilitate manipulation of the DOM server side and the ability to generate HTML and CSS dynamically, there are some key technologies and libraries that can be used with Deno to achieve this, which we will explore in this chapter.

We will be exploring fairly low-level enabling libraries and technologies, versus a full solution or framework for server side generation of websites.

JSX

Created by the React team at Facebook, JSX is a popular DSL (domain specific language) for embedding HTML-like syntax in JavaScript. The TypeScript team also added support for the JSX syntax into TypeScript. JSX has become popular with developers as it allows mixing imperative programming logic with a declarative syntax that looks a lot like HTML.

An example of what a JSX "component" might look like:

export function Greeting({ name }) {
  return (
    <div>
      <h1>Hello {name}!</h1>
    </div>
  );
}

The main challenge with JSX is that it isn't JavaScript nor is it HTML. It requires transpiling to pure JavaScript before it can be used in a browser, which then has to process that logic to manipulate the DOM in the browser. This is probably less efficient than having a browser render static HTML.

This is where Deno can play a role. Deno supports JSX in both .jsx and .tsx modules and combined with a JSX runtime, Deno can be used to dynamically generate HTML to be sent to a browser client, without the need of shipping the un-transpiled source file, or the JSX runtime library to the browser.

Read the Configuring JSX in Deno section for information on how to customize the configuration of JSX in Deno.

Document Object Model (DOM)

The DOM is the main way a user interface is provided in a browser, and it exposes a set of APIs that allow it to be manipulated via JavaScript, but also allows the direct use of HTML and CSS.

While Deno has a lot of web platform APIs, it does not support most of the DOM APIs related to visual representation. Having said that though, there are a few libraries that provide a lot of the APIs needed to take code that was designed to run in a web browser to be able to run under Deno, in order to generate HTML and CSS which can be shipped to a browser "pre-rendered". We will cover those in the following sections:

CSS

Cascading Style Sheets (CSS) provide styling for HTML within the DOM. There are tools which make working with CSS in a server side context easier and powerful.